Target discrimination in the extended shortwave infrared band (2 to 2.5μm) compared with visible, near-infrared, and SWIR in degraded visual environments

Abstract

.Long-range target identification is well studied in the visible (Vis) and near-infrared (NIR) bands and more recently in the shortwave infrared (SWIR). The longer wavelength of SWIR (1 to 1.7 μm) improves target detection for both long ranges and under challenging atmospheric conditions because it is less limited by scattering and absorption in the atmosphere. For these reasons, SWIR sensors are proliferating on military platforms. The extended shortwave infrared (eSWIR) band spanning from 2 to 2.5 μm is not typically limited by diffraction, and as a result, the band benefits target acquisition both at long ranges and for degraded visual environments (DVEs). Theoretical and experimental data compare eSWIR with Vis, NIR, and SWIR for atmospheric transmission, reflectivity, illumination, and sensor resolution and sensitivity. The experimental setup includes two testbeds, each with four cameras. The first is a wide field of view (FOV) testbed matching FOV at 20 deg for each camera. The second is a narrow FOV telescope testbed to match instantaneous FOV for consistent resolution across all four bands at long ranges. Both the theory and experiment demonstrate advantages of using eSWIR for long-range target identification under DVEs.

Comparison of reflective band (Vis, NIR, SWIR, eSWIR) performance in daytime reduced illumination conditions

“…Of the reflected band irradiance for the sun at nadir with no clouds, the Vis, NIR, SWIR, and eSWIR bands provide 0.0328, 0.0225, 0.0194, and 0.003 W/cm 2 respectively. 14 Considering the shortest and longest wavelength bands, eSWIR irradiance is about one tenth that of the Vis band. However, given that Vis photons are centered at roughly 0.5 µm, and eSWIR is roughly 2.25 µm in wavelength, there are around four times the number of photons in the eSWIR for the same energy flux [J/s or W].…”

“…9 reduction of scattering and absorption in the atmosphere, and better penetration through harsh atmospheric conditions, such as dust and smoke. 14 The scope of this paper is limited to non-intensified imagers because reflective band cameras are used extensively for long range target discrimination during the day. Passive image intensifiers are exclusively used at night under moonlight or starlight illumination conditions and can be damaged if exposed to daylight.…”

Target acquisition performance comparison of active continuous-wave and range-gated imaging systems at 1.6 and 2.1 μm

“…Three different atmospheric conditions are modeled; these correspond to MODTRAN’s US 1976 Standard Atmosphere with 23 and 5 km visibility and a hazy atmosphere (attenuation coefficients given in Refs. 13 and 14). Haze is the presence of very fine particles in the atmosphere that increase both scattering and the attenuation of light.…”